You can link against vdso.so if you want to; the kernel build provides an option to install that image. Doesn't mean that any particular libc uses it. On June 15, 2014 12:50:36 PM PDT, Ian Lance Taylor <iant@xxxxxxxxxx> wrote: >On Sun, Jun 15, 2014 at 12:31 PM, H. Peter Anvin <hpa@xxxxxxxxx> wrote: >> The weak symbols are well-known names. The __vdso symbols are >strong. > >I see. But I don't understand how this is supposed to work. When I >link a program against gettimeofday, I get a reference to gettimeofday >with version GLIBC_2.2.5. After all, I only link against libc.so; I >don't link against the vDSO. The VDSO provides gettimeofday with >version LINUX_2.6. Since those versions don't match, the gettimeofday >reference in my executable will not be satisfied by the definition in >the vDSO. So at dynamic link time my program is always going to be >linked with the gettimeofday in libc.so, which will in turn call the >gettimeofday in the vDSO. > >Am I missing something that makes the definition of gettimeofday with >version LINUX_2.6 in the vDSO useful? > >Ian > > > >> On June 15, 2014 12:22:17 PM PDT, Ian Lance Taylor <iant@xxxxxxxxxx> >wrote: >>>On Sun, Jun 15, 2014 at 12:14 PM, H. Peter Anvin <hpa@xxxxxxxxx> >wrote: >>>> >>>> If it doesn't, then you incur an additional indirection penalty. >The >>>strong __vdso symbol allows the libc wrapper to fall back to the vdso >>>implementation, the weak symbol allows three to be no wrapper at all. >>>This is good. >>>> >>>> The reason for changing ABI would be shifting types. This is very >>>much how glibc manages transitions. >>> >>>The purpose of symbol versioning is so that symbols with well known >>>names, like stat, can continue to use those same names while changing >>>types. Both old and new programs can continue to use the name stat >>>and continue to work even though they use different types. >>> >>>I don't see how this applies to the kernel VDSO. Those symbols do >not >>>use well-known names; they use names like __vdso_time. If you change >>>the types used by those symbols, you can change the name as well. >>>What is the downside? >>> >>>Ian >> >> -- >> Sent from my mobile phone. Please pardon brevity and lack of >formatting. -- Sent from my mobile phone. Please pardon brevity and lack of formatting. -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
